Method of preparing silver base alloys



United States Patent Int. Cl. C22c 5/00 US. Cl. 75173 Claims ABSTRACT OFTHE DISCLOSURE An improved method for preparing alloys for use informing electrical contact members consisting essentially of cadmiumoxide, silver and a reactive metal such as sodium involves the steps offirst heating a mixture of cadmium metal and silver to a temperaturewithin the range 1760 F. to 2100 F. to completely melt the mixture whilemaintaining a flux cover consisting of boric acid and rock salt on topof the mixture, adding the reactive metal carried, for example, withinthe end of a hollow carbon stirring rod through the flux cover andstirring the reactive metal into the melt for approximately 10 secondsto six minutes, and retaining the resulting melt under the flux coverfor at least approximately fifteen minutes. This latter step permits thereactive metal to diffuse throughout the melt to form a homogeneousmixture. The cadmium is subsequently internally oxidized to cadmiumoxide in the usual manner. By preparing the alloy in this manner, asubstantially homogeneous alloy is produced without excessive loss ofcadmium and/or the reactive metal through vaporization and thepercentage of reactive metal retained in the melt and the final alloymay thereby be more precisely controlled.

This invention lies in the field of alloys and more particularly relatesto improved methods of preparing silvercadmium oxide alloys for use informing electrical contact members.

U.S. Pats. Nos. 2,932,595, 3,472,654, and 3,477,845, and copending andcoassigned application Ser. No. 607,034, filed Jan. 3, 1967, describethe preparation of silver-cadmium oxide alloys containing additiveswhich regulate the grain size of silver-cadmium oxide particles andimpart improved properties to the alloys for their intended use inmaking electrical contact members therefrom. As described in the abovepatent and applications, the additives may be reactive metals such ascobalt, potassium, sodium, ruthenium, magnesium and beryllium. It hasbeen the practice to prepare such alloys by melting cadmium metal,silver and the reactive metal additive, with or without a flux cover, ata temperature within the range 1800 F.2100 F. and then pouring the meltto form an ingot. The ingot was then further processed as described tointernally oxidize the cadmium to cadmium oxide and to form electricalcontact members. While this method of preparation has been generallysatisfactory for the preparation of alloys containing reactive metaladditives such as cobalt and magnesium, for example, it has provengenerally unsatisfactory in the case of highly reactive metal additivessuch as sodium, potassium and lithium. Being highly reactive with theatmosphere and having lower melting points and boiling points than theother reactive metals mentioned above, vaporization causes excessiveloss of these metals, and it has not been possible with the methodsemployed heretofore to control with any degree of precision the amountof reactive metal retained in the melt and in the final alloy. Also,alloys of cadmium oxide, silver and sodium, potassium or lithiumprepared as described have not always exhibited the desired degree ofhomogeneity.

Among the objects of the invention, therefore, may be mentioned theprovision of improved methods for preparing silver base alloyscontaining sodium, potassium or lithium as a reactive metal additive;the provision of such methods which permit control of the amount ofreactive metal additive incorporated in the alloys and avoids excessivevaporization of the reactive metal and of cadmium during perparation ofthe alloys; and the provision of methods of this type which may bereadily carried out to produce alloys of substantially homogeneouscomposition. Other objects will be in part apparent and in part pointedout hereinafter.

The present invention is thus directed to the method of preparing analloy consisting essentially of cadmium oxide, silver and a reactivemetal such as sodium, potassium or lithium by first heating a mixture ofcadmium metal and silver to a temperature between approximately 1760 F.and 2100 F. to efiect substantially complete melting thereof whilemaintaining a flux cover on top of said mixture, adding the reactivemetal in at least a partially enclosed form through said flux cover andstirring the reactive metal into the melted mixture, retaining theresulting melt under the flux cover at said temperature for a period ofa least approximately fifteen minutes to diffuse the reactive metalthroughout the melt to form a substantially homogeneous mixture, andsubsequently internally oxidizing the cadmium to cadmium oxide.

In accordance with the present invention, it has now been found that bycarrying out the preparation of silver base alloys in the above manner,excessive vaporization and loss of the reactive metal is avoided and analloy of substantially homogeneous composition is obtained. Thus, I havefound that the shortcomings of prior art methods of preparing silverbase alloys with highly reactive metal additives therein may be obviatedby first effecting substantially complete melting of a cadmiummetal-silver mixture while maintaining a flux cover on top of themixture, adding the highly reactive metal in at least a partiallyenclosed form to the melted mixture through the flux cover and thenretaining the resulting melt under the flux cover for at least fifteenminutes, all steps being conducted at temperatures within the range 1760F. to 2100 F. The resulting alloy may then be cast into ingots, thecadmium internally oxidized to cadmium oxide and electrical contactmembers formed in the usual manner.

In carrying out the novel method of the invention, the desired amount ofcadmium metal is first loaded into the bottom of a crucible, such as asilicon carbide crucible, and covered with the required quantity ofsilver. A flux cover is then added over the cadmiumsilver mixture andthe resulting combination is heated to a temperature betweenapproximately 1760 F. and 2100 F. for a sufficient period to effectcomplete melting of the mixture. During heating, additional amounts offlux are added as needed to maintain a complete cover over the mixture.Various flux materials known to the art may be employed, and the use ofboric acid or a mixture of boric acid and rock salt has been found to beespecially advantageous. More particularly, mixtures of crystallineboric acid and rock salt in the range of ratios from approximately 2:1to 5:1 are preferred for use as the flux cover. The rock salt functionsto lower the viscosity of the boric acid and thereby causes the flux tomore readily flow over and cover the melting metal combination.

When the cadmium metal-silver mixture has become completely molten underthe flux cover, the highly reactive metal additive, such as sodium,potassium or lithium, is prepared for addition to the melt. Inaccordance with the invention, the reactive metal is added to the meltin a partially or wholly enclosed form so that excessive reactionbetween the metal and the atmosphere does not occur. Preferably, thereactive metal is first chopped into small pieces which are then stuffedinto the end of a hollow stirring rod of carbon or other inert material.In this partially enclosed form, the reactive metal carried by the rodis then inserted through the flux cover and into the bottom of thecrucible, and the melted. mixture stirred for a period of from aboutseconds to about six minutes to allow the reactive metal to work its wayout of the rod and into the melt. Less preferably, small pieces of thereactive metal may be enclosed or wrapped within thin films of silverand introduced through the flux cover and into the melt with stirring,as for example through the use of forceps composed of an inert material.In either case, the reactive metal is partially or wholly protected fromthe atmosphere and is introduced into the melt through the flux coverand thereafter stirred into the melt.

After the addition of the reactive metal to the melt as described, themelt containing the reactive metal additive therein is retained underthe flux cover at a temperature within the range previously stated(i.e., 1760 F. to 2l0O F.) for at least fifteen minutes and preferablyfor a period of from fifteen minutes to two hours. It has been foundthat retention of the melt under the flux cover for this period permitsthe reactive metal to diffuse throughout the melt to form asubstantially homogeneous mixture. In general, a one hour period ofretention of the melt under the flux cover is sufficient to effecthomogenization thereof at the preferred operating temperature of 1800-F-1900 F.

As stated, the method of the invention may be carried out at atemperature between approximately, 1760 F. and 2l0O F. For best results,it is preferred, however, to employ a temperature between approximately1800 F. and 1900 F. Temperatures within this range are sufficient toeffect complete melting of the cadmium metal-silver mixture while at thesame time avoiding excessive loss of cadmium through vaporiaztion.Similrly, after addition of the reactive metal to the molten mixture asdescribed, it is preferred to retain the resulting melt at a temperaturebetween approximately 1800 F. and 1900 F. to diffuse the reactive metalthroughout the melt. If higher temperatures within the range 1760F.-2100 F. are employed, then the period of retention is shortened toavoid excessive loss of cadmium and/ or of the reactive metal throughvaporization. Thus, for example, at temperatures within the preferredrange of 1800 Fl900 -F., one hour is the preferred period of retentionwhereas at higher temperatures such as 2050 'F,. a fifteen minute periodof retention would be adequate.

Following the homogenizing period, the melt is cast into molds andallowed to cool and set. After cooling, the molds are opened and theingots removed. For convenience, the ingots are quenched in water topermit easier handling. The impurities, if any, are present in the gateand butt ends of each ingot and these are there fore cut off anddiscarded. The ingots are then further processed in the conventionalmanner as described, for example, in US. Pat. No. 2,932,595, includinginternal oxidation of the cadmium to cadmium oxide and forming of thealloys into electrical contact members.

In general, the novel methods of the invention may be used in thepreparation of alloys consisting essentially of approximately 10%-20.0%by weight of cadmium oxide, approximately 0.00l%0.25% by weight ofsodium, potassium or lithium as the reactive metal additive, and thebalance silver. Preferably, the alloys may contain between approximately0.0025% and 0.0035% by weight (25 to 35 parts per million) of thereactive metal to impart the desired properties thereto.

By effecting substantially complete melting of the cadium metal-silvermixture prior to the addition of the reactive metal through the fluxcover as described and by retaining the melt, with the reactive metaltherein, under 4.- the flux cover for a sufficient period to permitdiffusion of the reactive metal throughout the melt, the percentage ofreactive metal retained may be more accurately controlled and an alloyof substantially homogeneous composition obtained.

The following examples further illustrate the invention.

EXAMPLE 1 An alloy consisting essentially of 10% by weight of cadmiumoxide, 0.0025%0.0035% by weight of sodium and the balance silver(approximately 90% by weight) was prepared as follows.

The bottom of a silicon carbide crucible was loaded with cadmium metaland the cadmium covered with the necessary quantity of fine silver grainand bars. A flux covered consisting of crystalline boric acid and rocksalt (ratio of 4:1) was then added on top of the cadmiumsilver mixture.The charge was then heated to a temperature of 1900 F. to effectcomplete melting of the cadmium-sil ver mixture, additional amounts offlux being added as necessary to maintain a complete flux cover duringheating of the mixture. Small pieces of sodium were stuffed into the endof a hollow carbon stirring rod and the end of the rod was then insertedinto the melted cadmium-silver mixture through the flux cover. Theamount of sodium added was 0.05% by weight of the cadmium-silver melt.The rod was inserted into the bottom of the crucible and the sodiumleaked out of the end of the rod and was stirred into the melt for aperiod of approximately three minutes. The rod was then withdrawn fromthe melt, and the melt maintained under the flux cover without stirringat a temperature of 1900 F. (+20 F., 50 F.) for a period of one hour.After the one hour period, the melt was cast into molds at a temperatureof 1900 F. under natural gas flame, After cooling, ingots of theresulting alloy were removed from the molds and washed. The gate andbutt ends of each ingot were cut off and discarded, and the ingots werethereafter processed in the conventional manner to internally oxidizethe cadmium to cadmium oxide and to form electrical contact members fromthe alloy.

EXAMPLE 2 Example 1 was repeated to prepare an alloy consistingessentially of 15% by weight of cadmium oxide, 0.0025% 0.0035% by weightof sodium and the balance silver (approximately The same procedure wasemployed except that the cadmium-silver mixture was melted at atemperature of 1800 F. and, after addition of the sodium, the resultingmelt was maintained at a temperature of 1800 F. (+20 F., 40 F.) for aperiod of one hour.

EXAMPLE 3 Example 1 was repeated except that potassium was employed asthe reactive metal in lieu of sodium.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantages results attained.

As various changes could be made in the above methods without departingfrom the scope of the invention, it is intended that all mattercontained in the above description shall be interpreted as illustrativeand not in a limiting sense.

What is claimed is:

1. The method of preparing a substantially homogeneous alloy consistingessentially of cadmium oxide, silver and a reactive metal selected fromthe group consisting of sodium, potassium and lithium which comprisesthe steps of heating a mixture of cadmium metal and silver to atemperature of between approximately 1760 F. and 2l0O F. to effectsubstantially complete melting thereof while maintaining a flux cover ontop of said mixture, adding said reactive metal in a least a partiallyenclosed form through said flux cover and stirring the reactive metalinto the melted mixture, retaining the resulting melt under said fluxcover at said temperature for a period of at least fifteen minutes todiffuse said reactive metal throughout the melt to form a substantiallyhomogeneous mixture, and subsequently internally oxidizing the cadmiumto cadmium oxide.

2. The method according to claim 1 wherein said flux cover consists of amixture of boric acid and rock salt.

3. The method according to claim 1 wherein said temperature is betweenapproximately 1800 F. and 1900 F. and the resulting melt is retainedunder said flux cover for a period of approximately one hour.

4. The method according to claim 1 wherein said reactive metal is addedthrough said flux cover by being carried in the end of a hollow stirringrod of inert material.

5. The method according to claim 4 wherein the resulting alloy containsbetween approximately 0.001% and 0.25 by weight of said reactive metal.

6. The method of preparing a substantially homogeneous alloy consistingessentially of approximately 1.0%- 20.0% by weight of cadmium oxide,approximately 0.001 %-0.25 by weight of a reactive metal selected fromthe group consisting of sodium, potassium and lithium, the balancesilver, which comprises the steps of heating a mixture of cadmium metaland silver to a temperature of between approximately 1800 F. and 1900 F.to effect substantially complete melting thereof, maintaining a fluxcover on top of said mixture during melting thereof, adding saidreactive metal through said flux cover and stirring the reactive metalinto the melted mixture, the reactive metal being carried in the hollowend of a stirring rod of inert material, retaining the resulting meltunder said flux cover at said temperature for a period of fromapproximately fifteen minutes to approximately two hours to ditfuse saidreactive metal throughout the melt to form a substantially homogeneousmixture, casting the melt into ingots and thereafter internallyoxidizing the cadmium to cadmium oxide.

7. The method according to claim 6 wherein said flux cover consists of amixture of boric acid and rock salt.

8. The method according to claim 7 wherein the ratio of boric to rocksalt in said flux cover is between 2:1 and 5:1.

9. The method according to claim 6 wherein said reactive metal issodium.

10. The method according to claim 9 wherein the resulting alloy containsbetween approximately 0.0025% and 0.0035% by weight of sodium.

References Cited UNITED STATES PATENTS 2,669,512 2/1954 Larsen et a1.173 2,932,595 4/1960 Pflumm 148l1.5 3,114,631 12/1963 Sistare et al.75-173 3,472,654 10/1969 Comey et al. 75173 3,477,845 11/1969 Comey etal 75-173 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, AssistantExaminer

